1. Field of the Invention
The present invention relates to an impedance matching apparatus and an endoscope including the same, and more particularly, to an impedance matching apparatus that achieves impedance matching in a channel when image pickup signals outputted from an image pickup device at an endoscope distal end portion are transmitted through a cable to a processor, and an endoscope including the impedance matching apparatus.
2. Description of the Related Art
Conventionally, a cable composing a channel from an image pickup device at an endoscope distal end portion to a processor has a length of about a few meters, so that impedance matching of the cable is important in view of a waveform grade. In recent years, a band of output signals from image pickup devices has become wide, and importance of impedance matching has further increased with it. However, cables have considerable impedance variations due to manufactural reasons, and waveform degradation caused thereby is a problem.
Conventional impedance matching methods will be described with reference to FIG. 18 through FIG. 20.
FIG. 18 schematically illustrates a channel of an endoscope. The endoscope includes an insertion portion to be inserted into a body cavity, an operation portion (not shown) connected to a proximal end side of the insertion portion, and a universal cable portion. The insertion portion of the endoscope has a distal end portion including a CCD as a solid image pickup device, a bending portion provided at a proximal end side of the distal end portion, and a flexible pipe portion having flexibility and being provided at a proximal end side of the bending portion. In the insertion portion, a signal cable, through which the CCD transmits and receives image pickup signals and power supply voltage, is inserted. The signal cable is additionally connected to a processor, not shown, via the operation portion and the universal cable portion.
In FIG. 18, reference numeral 10 denotes an endoscope distal end portion, reference numeral 20 denotes a cable having characteristic impedance ZO, and reference numeral 30 denotes a part of an analog front end portion.
The distal end portion 10 includes a CCD 11, a base resistance R1, an NPN transistor Q1 composing an emitter-follower, and an emitter resistance R2 as an output resistance. Collectors of the CCD 11 and the transistor Q1 are supplied with power supply voltage Vdd from outside.
The analog front end portion 30 includes a direct current termination resistance R3, a direct current cutting capacitor C1, an alternating current termination resistance R4, a preamplifier 32, and the like. The alternating current termination resistance R4 is composed of a variable resistance such as a trimming resistor that can be manually adjusted.
A condition of impedance matching is as follows: (an output resistance value of the transistor Q1)+(a resistance value of the resistance R2)=ZO=(a resistance value of the resistance R4).
Since variations in characteristic impedance ZO of the cable 20 can be smoothed by changing a value of the alternating current termination resistance R4, if a CCD output waveform is observed in transmission through the channel in FIG. 18 with the resistance R4 as a variable resistance that can be manually adjusted, a waveform as shown in FIG. 19 or FIG. 20 can be seen. A CCD output waveform can be seen by observing an outputted waveform from the analog front end portion 30 using a waveform observing apparatus.
FIG. 19 is a CCD output waveform with impedance matching obtained, and FIG. 20 is a CCD output waveform with impedance matching not being obtained. In FIG. 19, reference character f1 denotes a feedthrough part, f2 denotes a signal part, and f0 denotes a resetting portion. If impedance matching is not obtained, since a waveform in which reflected waves are superimposed on CCD output as shown in FIG. 20 is provided, such a waveform with impedance matching achieved as shown in FIG. 19 can be obtained by changing a value of the resistance R4 while a waveform of the CCD output is being observed.
Conventional arts related to a cable length of an endoscope are disclosed in, for example, Japanese Patent Application Laid-Open Publication Nos. 6-105807, 2006-055223, and 2001-016141.
Japanese Patent Application Laid-Open Publication No. 6-105807 discloses a signal processing apparatus of an electronic endoscope apparatus in which even if an electronic endoscope having a different length is used, without converting an operation timing, a correlated double sampling circuit and the like are effectively operated as well as a circuit configuration is simplified, which facilitates handling.
Japanese Patent Application Laid-Open Publication No. 2006-055223 discloses an endoscope whose signal connector includes a connector substrate on which a signal pattern for transmitting an output signal from a CCD apart from a drive circuit is provided in order to prevent a drive signal from mixing as a noise. Thereby, influence of noise owing to a drive signal can be reduced and even if a type of a solid image pickup device is different, it is easy to apply the endoscope thereto.
Japanese Patent Application Laid-Open Publication No. 2001-016141 discloses a cable length compensating apparatus for compensating an influence on signal resolution owing to a length of a signal cable used if a video imaging system such as an X-ray video imaging system is set up, the compensation being achieved by the cable compensating apparatus being installable in a signal path along a cable and compensating a signal for the influence of the cable to provide a desired gain across a desired range of a signal frequency.